The first mode of the invention will be now described.
In manufacturing electronic devices, wire bonding technology has been utilized in packaging an LSI chip or chips on a semiconductor substrate. The electronic devices are provided with pads for electrical connection of the devices with external lead wires which are made of a material containing aluminum as a major composition. So-called tape Automated Bonding (TAB) technology and Flip Chip technology are also used in packaging LSIs. All of these techniques commonly utilize bumped electrodes which are formed on the pads.
It is noted that these pads must be positioned in an adequate arrangement suitable not only for a design of the LSI but also for a particular bonding technique used, since the arrangement of such pads generally depends on the type of the packaging technology used.
This implies that even those LSI chips having an identical basic structure and identical functions must be packaged in different ways if they are packaged by different techniques. They would then require different packaging designs and additional product management as if they were different LSIs. This would in turn add undesirable complexity and increased cost to the management of the products, thereby rendering the final LSIs costly.
Accordingly, it would be advantageous to provide chips that may be equally packaged by different types of packaging techniques, thereby unaffected by the disadvantages mentioned above.
One way to achieve this feature is disclosed in Japanese Patent Publication Laid Open Nos. 2-121333 and 5-218042, in which the chip is provided with primary pads at positions independent of packaging techniques, and the pads are rearranged in accordance with the packaging technique employed.
Briefly, the pad rearrangement techniques as disclosed in the foregoing patents are based on the formula in which lead wires are formed on a protective insulation substrate having openings or through-holes for primary pads, and additional or secondary pads are mounted on the lead wires extending between the primary pads and secondary pad sites away from the primary pads, so that external lead wires may be connected to the secondary pads at arbitrary positions.
Briefly, these pad rearrangement techniques are based on the formula that additional pads or secondary pads are mounted on lead wires at arbitrary positions away from the original pads so that external lead wires may be connected at those arbitrary positions. These secondary pads are formed on a protective insulation substrate laid on the substrate and having openings for the primary pads.
In the prior art techniques mentioned above, the lead wires have a multi-layered lead structure, which consists of, for example, three layers of titanium (Ti), nickel (Ni), and gold (Au), or two layers of titanium and copper (Cu), deposited in the order mentioned.
Similar prior art techniques for multi-layered lead wires have been also disclosed in Japanese Patent Publication Laid Open Nos. 60-136339, 57-122542, 62-183134, and 1-290232.
The first of these publication concerns with Ti--Cu--Ni layers; the second with Ti--Cu--Ti layers; the third with Ti--Pd--Ti layers; and the last with Al--V--Al layers and Al--Ti--Al layers.
It should be kept in mind, however, that the secondary pads must be necessarily located at sites above the corresponding active semiconductor circuit element (or active component), and hence that the active component must be protected from the weight of the pads plus the bonding load acting on the active component during bonding of the external lead wires onto the secondary pads.
It should be also kept in mind that the sheet resistance of the lead wires must be sufficiently low, since otherwise the lead wires acquire high resistance due to the fact that the secondary pads must be properly spaced apart from their primary pads, so that the lead wires extending between them can be long.
Further, it should be noted that the secondary pads must have good adhesive power to bumps to be formed on the pads.
Still further, the rearrangement structure of the lead wires for the secondary pads must have a structure that may prevent electromigration between them, especially in the case where the chip density of the LSI is increased and accordingly the spacing between the lead wires is decreased. Particularly, the lead wires containing Cu, Au or Ag must have a structure immune to such electromigration.
However, any of the prior art lead wire structures can not satisfy these requirements altogether. There is accordingly a need for a semiconductor device having a novel lead wire structure which meets all the requirements.
The second mode of the invention will be now described.
There have been known many types of techniques for mounting semiconductor chips or electronic devices such as LSIs on a printed circuit board. Choice of an appropriate technique depends on the particular terminal arrangement of the electronic devices to be mounted.
A well known terminal arrangement for electronic devices like LSIs is a peripheral type terminal arrangement, in which terminals extend outwardly from the periphery of the package. A typical example of this type is a quad flat package (QFP).
However, in order to meet a recent requirement for miniaturization of electronic devices and for high density packaging thereof, a mounting technique utilizing so called "face down" solder bumps has been frequently used because these bumps permit of external connection of the terminals and occupy a relatively small mounting area. The solder bumps are mounted on bump lands or terminals provided on an insulation layer. Thus, each of the bump lands needs a predetermined area. Now that an electronic device such as LSI has a very large number of gates, it necessarily has many input and output terminals. Therefore, the electronic device is mounted using a bump grid array in which the bump lands are arranged in the form of generally planar grid in order to arrange many bump lands neatly on the insulation layer. This type of bump structure is also called area array bump structure.
An example of such area array bump structure is shown in FIG. 6A, which shows in a partial side elevational cross section a major portion of a semiconductor device having a conventional solder bump structure. Formed together with various bonding pads and lines on an insulation substrate 52 of a material such as polyimide, is a plurality of bump lands 53 for mounting thereon solder bumps.
A semiconductor chip such as LSI (not shown) is die-bonded at a predetermined position on the top surface of the insulating material 52, using appropriate glue.
The terminals of the semiconductor chip and the bonding pads (not shown) formed on the insulation substrate 52 are electrically connected as needed by means of, for example, an appropriate wire bonding technology. In order to increase the reliability of the electronic device, it is coated with a sealing resin 51.
Each of the bump land 53 is provided with a small through-hole or opening 55 formed in the insulation substrate 52, which is smaller in area compared to the bump land 53. Solder bumps 54 are bonded on the bump lands 53 in the respective openings 55 by overturning the insulation substrate 52, placing the solder bumps 54 in the respective through-holes 55, and applying solder re-flow treatment.
The diameter of each through-hole is constant throughout the thickness of the insulation substrate 52. Thus, the configuration of the through-holes for accommodating the solder bumps 54 is generally cylindrical. That portion of the solder over-flown out of each through-hole has a generally round shape due to the surface tension. This is how solder bumps are formed in packaging a semiconductor device.
In conventional packaging methods using an area array bump structure as mentioned above, connection failures can happen when and after a semiconductor device is bonded to a printed circuit board and installed further in an electronic equipment.
The inventors of the present invention have examined many of such connection failures associated with the area array bump structures and have found that most of the connection failures are caused by the exfoliation (indicated by reference number 42) of the solder bump lands 53 of the semiconductor devices from the solder bumps 54 as shown in FIG. 6B.
The inventors have found that such exfoliation of the bump lands 53 from the solder bumps 54 is caused by an excessive external stress that arises from handling of the semiconductor device or the printed circuit board 61, installing the printed circuit board 61 to the electronic equipment, and mismatching of thermal expansions of the insulation layer 52 accommodating the bumps and the printed circuit board 61, and that concentrate in the bonding interfaces between the solder bumps 54 and the bump lands 53.
The inventors have reached a conclusion that such exfoliation of the bump lands 53 and the solder bumps 54 can be avoided if the solder bumps are provided with mechanically weak portions that may deform to absorb the external stresses. The invention is based on this principle.
It might seem a common practice to make the initial configuration of the solder bumps in the through-holes in any desired form, for example a straight cylindrical shape, a barrel shape having a diametrically larger middle section, or a "bongo" shape having a diametrically smaller middle section, in accord with the configuration of the through-holes formed in the insulation substrate. It should be appreciated, however, that positive provision of the bump configuration having such mechanically weak portions is based on a novel and useful findings of the inventors, which is useful in avoiding the exfoliation of the solder bumps by concentrating external stresses at the weak portions of the solder bumps, thereby absorbing such stresses applied to the printed circuit board and the semiconductor devices during and after mounting the semiconductor devices on the printed circuit board.